1. Field of the Invention
The present invention relates to a display device, and more particularly, to a backlight device of a liquid crystal display device.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device includes an LCD panel having a liquid crystal layer between two substrates, a driving circuit for driving the LCD panel, and a backlight device functioning to provide light to the LCD panel. The two substrates of the LCD panel includes a lower substrate, which has a thin film transistor array (TFT) array formed thereon. The lower substrate is bonded to an upper substrate, which has a color filter array formed thereon. The upper substrate is bonded to and separated from the lower substrate by a predetermined interval, wherein the liquid crystal layer is formed within the predetermined interval between the lower and upper substrates. Accordingly, images are displayed by controlling transmissivity of the light produced by the backlight device according to a voltage applied to a plurality of pixels.
Since the LCD panel does not produce light by itself, the LCD panel uses either ambient light or an additional light source, i.e., a backlight device Accordingly, the LCD panel is classified as transmitting-type and reflective-type LCD panels according to the type of light source. For example, the transmitting-type LCD panel uses the additional light source, whereas the reflective-type LCD panel uses the ambient light. In addition, the transflective-type LCD panel may use both the additional light source and the ambient light.
Backlight devices are commonly required to emit intense amounts of light while at the same time minimumizing power consumption. The backlight devices are classified as direct-type and edge-type according to a position of a fluorescent lamp. In the direct-type backlight device, the fluorescent lamp emits light to an entire rear side of the LCD panel. In the edge-type backlight device, the fluorescent lamp is formed at an edge of the LCD panel and the light emitted from the fluorescent lamp is provided at the rear side of the LCD panel through a light-guiding plate.
In the direct-type backlight device, a plurality of fluorescent lamps may be formed below the LCD panel, or a single fluorescent lamp having a bent portion may be formed. Accordingly, a predetermined interval must be maintained between the fluorescent lamps and the LCD panel in order to prevent silhouettes of the fluorescent lamps from being projected onto the LCD panel. In addition, a light-scattering system is formed to provide uniform light intensity onto the LCD panel. Thus, it is difficult to obtain a low profile LCD device when using the direct-type backlight device.
In the edge-type backlight device, since a light-guiding plate is used for uniformly scattering the light onto an entire surface of the LCD panel, luminance is low. Accordingly, the edge-type backlight device is used in LCD devices requiring low profiles, such as notebook computers, and the direct-type backlight device is used in LCD devices requiring large-sized screens and high luminance.
FIG. 1 is a cross sectional view of a liquid crystal display device having a direct-type backlight device according to the related art. In FIG. 1, an LCD device includes an LCD panel 1, a plurality of fluorescent lamps 5, a light-scattering system 2, a reflecting plate 4, a plurality of supports 6, and a case 9. The LCD panel displays images by controlling transmissivity of light produced by the plurality of fluorescent lamps 5 formed below the LCD panel at fixed intervals. In addition, the light-scattering system 2 is formed between the LCD panel 1 and the plurality of fluorescent lamps 5 to prevent silhouettes of the plurality of fluorescent lamps 5 from being projected onto a display surface of the LCD panel 1, and to provide a uniform limunance of the light produced by the plurality of fluorescent lamps 5. The plurality of supports 6 provide support to the light-scattering system 2. The reflecting plate 4 is formed below the plurality of fluorescent lamps 5 in order to reflect the light emitted from the plurality of fluorescent lamps 5 onto the LCD panel 1, wherein the case 9 supports the plurality of fluorescent lamps 5 and the reflecting plate 4.
FIG. 2 is a perspective view of a direct-type backlight device according to the related art. In FIG. 2, a direct-type backlight device includes a plurality of fluorescent lamps 5 formed at fixed intervals, a case 9 for fixing and supporting the plurality of fluorescent lamps 5, a light-scattering system 2a, 2b, and 2c formed between the LCD panel 1 and the plurality of fluorescent lamps 5, and a reflecting plate 4 formed on an inner side of the case 9 to concentrate the light emitted from the plurality of fluorescent lamps 5 onto a display part of the LCD panel. The light-scattering system 2a, 2b, and 2c includes a plurality of diffusion sheets and plates, and the reflecting plate 4 and the case 9 are formed of a material having a high heat conductivity, such as Al materials. In addition, the plurality of fluorescent lamps 5 are cold cathode fluorescent lamps (CCFL), wherein electrodes are formed at both ends of the lamps 5. Accordingly, the plurality of fluorescent lamps 5 emit light when power is supplied to the electrodes at both ends of the plurality of fluorescent lamps 5, wherein the ends of the plurality of fluorescent lamps 5 are inserted into holes formed at both sides of the case 9.
When the plurality of fluorescent lamps 5 emit the light, a large portion of the emitted light is directly incident onto the light-scattering system 2a, 2b, and 2c, and a small portion of the emitted light is reflected by the reflecting plate 4 and redirected onto the light-scattering system 2a, 2b, and 2c. The light-scattering system 2a, 2b, and 2c scatters the incident light to provide uniformity onto the LCD panel 1, thereby displaying images.
When the light emitted from the plurality of fluorescent lamps 5 is incident onto the LCD panel 1, the plurality of fluorescent lamps 5 produce significant amounts of heat. Most of the heat emitted from the fluorescent lamp 5 is transmitted to the LCD panel 1 through the light-scattering system 2a, 2b, and 2c. However, some of the heat is dissipated to the surrounding by being absorbed by the reflecting plate 4 and transmitted to the case 9.
However, the direct-type backlight device has the following disadvantages. First, as a size of the LCD panel 1 increases, the number of fluorescent lamps 5 increases, thereby increasing the amount of heat produced by the plurality of fluorescent lamps 5. Second, as the amount of heat produced by the plurality of fluorescent lamps 5 increases, the lifetime of the plurality of fluorescent lamps 5 decreases. Third, the amount of heat produced by the plurality of fluorescent lamps 5 adversely affects the LCD panel 1.
One solution to reduce the amount of heat produced by the-plurality of fluorescent lamps 5 involves using a cooling fan. However, the cooling fan increases the unit manufacturing costs of the LCD device, increases power consumption of the LCD device, and generates noise.